The polymerization of ethylene, propylene and other α-olefins is a market in excess of 100 million tons and is growing at an annual rate of about 5 percent. The catalyst industry that supports the polyolefin industry has sales approaching a billion dollars in the US alone. The catalyst market is presently dominated by Ziegler/Natta catalysts with metallocene and other single-site catalysts growing as a proportion of the market. Most olefin polymerization catalysts are solid supported catalysts comprising titanium compounds in combination with cocatalysts, which are commonly organoaluminum. Other olefin polymerization catalysts include chromium catalysts, which can be chromium oxides on silica or silica alumina, the Phillips process, and supported chromacene, a metallocene based catalyst used in the Union Carbide process. Other common metallocene catalysts include mono- and bis-metallocene complexes of Ti, Zr or Hf, which are used commonly in combination with a cocatalyst, such as methylaluminoxane (MAO). Non-metallocene catalysts are complexes of various transition metals with other multidentate oxygen-based and nitrogen-based ligands and are typically combined with MAO or other cocatalyst.
The development of catalysts that do not require MAO or other expensive cocatalysts as an activator has proceeded slowly and few examples are found in the literature. For example, Bhandari et al., Organometallics 1995, 14, 738-45 discloses (benzyl)chromium (III) complexes that polymerize ethylene alone, or with a co-catalyst. Ajjou et al., J. Am. Chem. Soc. 1998, 120, 415-416 discloses a bis(neopentyl)chromium(IV) surface complex on amorphous silica that polymerizes ethylene and propylene. Heintz et al., Organometallics 1998, 17, 5477-5485 discloses Bis(trimethylsilylmethyl)(η5-pentamethylcyclopentadienyl) chromium(III) for the polymerization of ethylene. MacAdams et al. J. Amer. Chem. Soc., 2005, 127, 1082-3 discloses [(2,6-Me2Ph)2nacnacCr(OEt2)CH2SiMe3]BARF where nacnac=2,4-pentane-N,N′-bis(aryl)ketiminato and BARF=B(3,5-(CF3)2C6H3)4− for the polymerization of ethylene. The absence of MAO presents possibilities for control of rates, stereochemistry, molecular weights, and molecular weight distributions that are easily achieved by the inclusion of a single catalytic species in a pure form. To this end, the development of single component catalysts or a catalyst that permits the use of an inexpensive activator or cocatalyst may result in improved polymers and/or polymerization processes for polyethylene and polypropylene.
Additionally, catalysts that polymerize ethylene have the potential for use as olefin isomerization catalysts. Isomerization of functionalized substrates can be catalyzed by transition metal complexes, and the isomerization of olefins is a very important type of reaction commercially. Specifically, 1-alkene isomerization is a key step in many industrial processes, particularly in petrochemical refining, and selective olefin isomerization under mild conditions is an important goal.
Transition metal catalyzed isomerization of terminal olefins to internal olefins has been accomplished by Ir, Ru, and Rh metal hydrides or by complexes that can be converted into metal hydrides in the presence of a proton source. Generally, these catalytic systems operate via a metal-hydride addition-elimination pathway with a 1,2-hydrogen migration, although Fe and Ru carbonyl and phosphine substituted carbonyl derivatives have been shown to isomerize 1-alkenes through a π-allyl hydride intermediate that results in a 1,3-hydrogen shift.
Isomerization from terminal to internal alkenes is thermodynamically favorable because of the higher stability of internal olefins, and equilibration of alkenes favors the formation of structures where, when possible, the double bond is far from the end of the carbon chain. Generally, known catalysts yield a thermodynamic equilibrium mixture of isomeric alkenes with little selectivity. High selectivity for the isomerization of 1-alkenes to 2-alkenes has been achieved when a conjugated diene product is formed, but only a few examples of highly selective olefin isomerization catalysts exist that do not depend on the formation of a conjugated product. For example, organotitaniums, Bergbreiter et al., J. Organomet. Chem. 1981, 208, 47, or titanocene and zirconocene alkyne derivatives, Ohff et al., J Mol. Catal. 1996, 105, 103, have been disclosed to selectively isomerize 1-alkenes to their corresponding 2-alkenes. Therefore, catalysts that can effectively isomerize 1-alkenes to internal alkenes with selectivity are desirable.